This invention relates to a magnetic disk substrate used for a hard magnetic disk recording device and, more particularly, to a magnetic disk substrate made of a glass-ceramic suitable for use as a contact recording type magnetic disk.
There is an increasing tendency for utilizing a personal computer for multiple media purposes and this tendency necessitates a hard magnetic disk device of a larger recording capacity. For this purpose, the bit number and track density of a magnetic disk must be increased and the size of a bit cell must be reduced for increasing a surface recording density. As for a magnetic head, it must be operated in closer proximity to the disk surface in conformity with the size reduction of the bit cell and, for this purpose, the contact recording system has been developed according to which recording is performed with the magnetic head being nearly in contact with the disk surface, and with the amount of lifting of the magnetic head from the disk surface being maintained below 0.025 .mu.m.
In such contact recording system, the magnetic disk is required to have the surface roughness (Ra) of 10 .ANG. or below in order to maintain the amount of lifting of the magnetic head below 0.025 .mu.m. Besides, the magnetic disk is required to have abrasion resistance which is sufficiently high for standing contact with the magnetic head.
Aluminum alloy has been conventionally used as a material of magnetic disk substrate. The aluminum alloy substrate, however, tends to produce a substrate surface having projections or spot-like projections and depressions during the polishing process due to defects inherent in the material. As a result, the aluminum alloy substrate is not sufficient in flatness. Further, since an aluminum alloy is a soft material, deformation tends to take place so that it cannot cope with the recent requirement for making the magnetic disk thinner and the requirement for high density recording because the disk tends to be deformed by contact with the head with resulting damage to the recorded contents.
As a material for overcoming this problem of the aluminum alloy substrate, known in the art are glass substrate for magnetic disks made of a chemically tempered glass such as a sodium lime glass (SiO.sub.2 --CaO--Na.sub.2 O) and alumino-silicate glass (SiO.sub.2 --Al.sub.2 O.sub.3 --Na.sub.2 O). This glass substrate, however, has the following disadvantages:
(1) Polishing is made after chemical tempering and so the tempered layer tends to cause instability in thinning the disk substrate. PA0 (2) Since the Na.sub.2 O ingredient is included as an essential ingredient in the glass, the film forming characteristic of the glass is deteriorated with the result that a surface coating processing becomes necessary for preventing eluting of the Na.sub.2 O ingredient. This prevents mass production of the product at a competitive cost.
Aside from the aluminum alloy substrate and chemically tempered glass substrate, known in the art are some substrate made of glass-ceramics. For example, Japanese Patent Application Laid-open No. 6-329440 discloses a glass-ceramic of a SiO.sub.2 --Li.sub.2 O--MgO--P.sub.2 O.sub.5 system which includes lithium disilicate (Li.sub.2 O.2SiO.sub.2) and alpha-quartz (alpha-SiO.sub.2) as predominant crystal phases. This glass-ceramic is an excellent material in that the grain size of globular grains of alpha-quartz can be controlled but surface roughness (Ra) of a polished surface is within a range from 15 .ANG. to 50 .ANG.. This glass-ceramic cannot cope with the above described target surface roughness (Ra) of 10 .ANG. or below so that it cannot sufficiently cope with the above described tendency to lowering of the amount of lifting of a magnetic head necessitated by rapid increase in the recording capacity.
It is, therefore, an object of the invention to eliminate the above decsribed disadvantages of the prior art substrates and provide a glass-ceramic substrate for a magnetic disk which is suitable for use as a contact recording type magnetic disk for coping with the tendency toward high recording density of a hard magnetic disk by having surface roughness (Ra) of 10 .ANG. or below and also having a high abrasion resistance.